Sub-scanning interval adjusting apparatus for multi-beam laser scanning unit

ABSTRACT

A sub-scanning interval adjusting apparatus for a multi-beam laser scanning unit adjusts an interval between at least two laser lines that are formed on a photoreceptor drum without causing starting points of image formation to vary. In the multi-beam laser scanning unit simultaneously emitting at least two laser beams by using at least two laser sources, the sub-scanning interval adjusting apparatus includes a transparent member varying in thickness depending on its height, in a direction where the laser beams pass through, the transparent member having an inclined lower side with respect to a scanning direction of the laser beams, and a movable member having an inclined side corresponding to the inclined lower side of the transparent member, thereby adjusting an interval between the laser beams by moving the movable member in the scanning direction and thus varying the height of the transparent member with respect to the laser sources. An elastic member is formed on the transparent member to press the transparent member with respect to the movable member. The transparent member is formed to have a shape of a triangle or a trapezoid in cross-section in the direction where the laser beams pass through. The transparent member has a refractivity greater than 1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.2002-49961, filed Aug. 23, 2002, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a multi-beam laser scanningunit used in an image forming apparatus, such as a laser printer, andmore particularly, to a sub-scanning interval adjusting apparatus of amulti-beam laser scanning unit adjusting a distance between at least twobeams of the multi-beam laser scanning unit.

2. Description of the Related Art

Generally, a laser printer forms an image by processes of processes offocusing a laser beam from a laser scanning unit onto a photoreceptordrum in accordance with a video signal, developing an electrostaticlatent image on the photoreceptor drum, and transferring the developedimage onto a printing medium such as a paper. An exemplary structure ofthe laser scanning unit for use in the laser printer is shown in FIG. 1.

Referring to FIG. 1, the laser scanning unit is provided with a lasersource 10, a collimating lens 11, a cylinder lens 12, a polygon mirror13, an f-theta lens 15, and a reflective mirror 16.

The laser source 10 generates a laser beam 1, and usually a laser diodeis used as the laser source.

The collimating lens 11 transforms the laser beam 1 from the laser diode10 into a parallel or convergence ray of light (laser beam 1 havingparallel rays) with respect to an optical axis.

The cylinder lens 12 focuses the laser beam 1 passed through thecollimating lens 11 onto a surface of the polygon mirror 13 in a form ofa line in a horizontal direction.

The polygon mirror 13 is rotated by a motor 14 at a constant speed toperform scanning by displacing the laser beam 1 passed through thecylinder lens 12 with a constant linear velocity in the horizontaldirection.

The f-theta lens 15 has a predetermined refractivity with respect to theoptical axis, deflects the laser beam 1 reflected from the polygonmirror 13 at constant velocity towards the main scanning direction(arrow A), and compensates for aberrations, thereby focusing the laserbeam 1 on the scanning surface.

The reflective mirror 16 reflects the laser beam 1 passed through thef-theta lens 15 towards a certain direction, thereby focusing the laserbeam 1 on a surface of a photoreceptor drum 17 in a form of a dot.

Further provided is an optical sensor 19 for horizontal-synchronizingthe laser beam 1. The optical sensor 19 is provided with asynchronization signal detecting reflective mirror 18, which reflectsthe light passed through the f-theta lens 15 towards the optical sensor19.

The above-mentioned components are assembled in a single frame (notshown) to form the laser scanning unit. Further, a transparent glassmember (not shown) is attached to an outer side of the frame to preventforeign substances, such as dust, from being introduced into the laserscanning unit.

Increasingly, demands on the laser printer that prints at a higherprinting speed have been growing. In order to increase the printingspeed of the laser printer, a scanning speed of the polygon mirror 13needs to be increased. And in order to increase the scanning speed, anR.P.M of the motor 11 has been increased to increase the R.P.M of thepolygon mirror 13. This way, however, has been accompanied by problems,such as an increase of noise, and also a high material cost mainly dueto the requirement for a high-speed motor.

In an attempt to solve the above-mentioned problems, recently employedis a multi-beam laser scanning unit that has a plurality of lightsources. In this case, if the multi-beam laser scanning unit having twolight sources is employed to guarantee the constant printing speed, themotor 14 for the polygon mirror 13 has a half R.P.M of a general motor.

With the multi-beam laser scanning unit, two lines of a given resolutionare simultaneously formed on the photoreceptor drum 17 in aperpendicular direction (hereinafter called ‘sub-scanning direction’) inrelation to the main scanning direction.

In the meantime, when components of the laser scanning unit areassembled, errors inevitably occur due to assembling status or precisionof the respective components.

Accordingly, a space interval between two lines in the sub-scanningdirection is deviated from a normal interval that is predetermined forthe corresponding resolution. And this usually causes an undesiredphenomenon, so-called ‘jittering’, in the laser printer withhigh-resolution, deteriorating a sharpness of a printing image.

In order to solve the above-mentioned problems, as shown in FIG. 2, anapparatus 20 is provided to adjust an interval between multi-beams inthe sub-scanning direction.

Referring to FIG. 2, the multi-beam interval adjusting apparatus 20includes a substrate 21 and a sub-assembling plate 23.

The substrate 21 is mounted with a laser diode 22 as a light source, anda circuit driving the laser diode 22. The sub-assembling plate 23 has alens barrel 23-1 formed at a center thereof, and a plurality ofelliptical holes 23-3 allowing the sub-assembling plate 23 to rotateabout the lens barrel 23-1 by a predetermined angle. Assembled in thelens barrel 23-1 is a collimating lens 24. A tap hole 23-2 is formed onthe sub-assembling plate 23 to secure the substrate 21 on the assemblingplate 23. The substrate 21 is assembled with respect to thesub-assembling plate 23 by screws 26 through screw holes 21-1. Then,after the lens barrel 23-1 of the sub-assembling plate 23 is inserted ina hole 25-1 formed on the frame 25, the sub-assembling plate 23 isfastened to the frame 25 with screws 27. The screws 27 are fastened tothe frame 25 through the elliptical holes 23-3 of the sub-assemblingplate 23 and screw holes 25-2 of the frame 25. Accordingly, the lensbarrel 23-1 of the sub-assembling plate 23 is adjustable in a preciseposition with respect to the hole 25-1 of the frame 25.

Accordingly, when laser light (a laser beam) is emitted from the laserdiode 22, the laser beam passes through the lens barrel 23-1 and isincident on the collimating lens 24 and concentrated thereon in a formof a parallel ray.

In a case that the two lines of the laser beam concentrated on thephotoreceptor drum 17 in the sub-scanning direction are disposed at aninterval while being deviated from a predetermined interval, suchdeviation is adjusted by minutely moving and fastening thesub-assembling plate 23 on the frame using the screws 27.

However, the multi-beam interval adjusting apparatus 20 as describedabove has a shortcoming. That is, the image formation of the two laserlines in the scanning direction starts from different positions. By wayof one example, FIG. 3A shows two laser diodes 22-1, 22-2 that areprovided on the same line and in perpendicular relation with respect tothe scanning direction, and at a predetermined interval (p). As arepredetermined, the two lines are formed on the photoreceptor drum 17 atthe predetermined interval (p). Then when the two lines are formed at aninterval other than the predetermined interval (p), such deviation isadjusted by adjusting the interval between the laser diodes.Accordingly, the sub-assembling plate 23 is rotated to adjust theinterval between the two lines as predetermined. However, while theinterval between the two laser diodes 22-1, 22-2 in the sub-scanningdirection becomes p′, the laser diodes 22-1, 22-2 are deviated from apredetermined plane in the main scanning direction by a distance (Δ)(see FIG. 3B). As a result, the image formation in the scanningdirection starts differently. Such a problem can be solved by employinga software or device for compensation.

However, employment of the compensation software or device causes astructure of the laser scanning unit to be complex, and a material costincreases.

SUMMARY OF THE INVENTION

Accordingly, it is an aspect of the present invention to provide asub-scanning interval adjusting apparatus of a multi-beam laser scanningunit adjusting an interval in a sub-scanning direction between two linesof laser light with ease without having deviation of starting positionsof the laser light of image formation in a scanning direction.

Additional aspects and advantages of the invention will be set forth inpart in the description which follows and, in part, will be obvious fromthe description, or may be learned by practice of the invention.

In order to accomplish the above and/or other aspects of the invention,a sub-scanning interval adjusting apparatus of a multi-beam laserscanning unit adjusts an interval between at least two laser beams usingthe laser beams being simultaneously emitted from the at least two lasersources of the multi-beam laser scanning unit and includes a transparentmember arranged on an optical path of the laser beams to adjust theinterval between the laser beams.

The transparent member varies in thickness depending on its height in adirection where the laser beams pass through, and is either a triangleor a trapezoid in cross-section in the direction where the laser beamspass through.

According to another aspect of the present invention, a sub-scanninginterval adjusting apparatus of a multi-beam laser scanning unitadjusting an interval between at least two laser beams uses the laserbeams being simultaneously emitted from the at least two laser sourcesof the multi-beam laser scanning unit and includes a transparent membervarying in thickness depending on its height in a direction where thelaser beams pass through, and a movable member adjusting the transparentmember in height. Accordingly, the interval between the laser beams isadjusted by adjusting the height of the transparent member with respectto the laser sources using the movable member.

According to another aspect of the present invention, an elastic memberis formed on the transparent member to press the transparent member.

According to another aspect of the present invention, a sub-scanninginterval adjusting apparatus of a multi-beam laser scanning unitadjusting an interval between at least two laser beams uses the laserbeams being simultaneously emitted from at least two laser sources ofthe multi-beam laser scanning unit and includes a transparent membervarying in thickness depending on its height in a scanning directionwhere the laser beams pass through, the transparent member having aninclined lower side towards the scanning direction of the laser beams,and a movable member having an inclined side corresponding to theinclined lower side of the transparent member. Accordingly, the intervalbetween the laser beams is adjusted when the movable member movestowards the scanning direction and thus adjusts the height of thetransparent member with respect to the laser sources.

According to another aspect of the invention, the transparent member istriangular or trapezoidal in cross-section in the direction where thelaser beams pass through. The transparent member has an opticalrefractivity great than 1.

With a sub-scanning interval adjusting apparatus for a multi-beam laserscanning unit according to the present invention, an interval between aplurality of laser beams in a sub-scanning direction can be easilyadjusted without causing starting points of image formation in ascanning direction to be deviated from each other. Since there is noneed to employ software or a device to compensate for the deviation ofthe image formation starting points, construction of the scanning unitbecomes simplified, and a cost of a material is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe preferred embodiments, taken in conjunction with the accompanyingdrawings of which:

FIG. 1 is a view showing a structure of a conventional laser scanningunit;

FIG. 2 is an exploded perspective view of an apparatus for adjusting aninterval in a sub-scanning direction for the conventional multi-beamlaser scanning unit of FIG. 1;

FIGS. 3A and 3B are views showing deviations in starting points of imageformation due to an interval adjustment of the interval adjustingapparatus of FIG. 2;

FIG. 4 is a view showing an apparatus for adjusting an interval in asub-scanning direction for a multi-beam laser scanning unit according toan embodiment of the present invention;

FIG. 5 is a side view of the interval adjusting apparatus of FIG. 4;

FIG. 6 is a view showing a change in an optical path of a laser beampassing through a transparent member of the interval adjusting apparatusof FIG. 4;

FIG. 7A is a view showing a change in the optical path of two parallellaser beams passing through the transparent member of the intervaladjusting apparatus of FIG. 4;

FIG. 7B is a view showing a change in the optical path of twonon-parallel laser beams passing through the transparent member of theinterval adjusting apparatus of FIG. 4;

FIG. 8 is a view showing another apparatus for adjusting an interval ina sub-scanning direction for a multi-beam laser scanning unit accordingto another embodiment of the present invention; and

FIG. 9 is a side view of the interval adjusting apparatus of FIG. 8.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described in order to explain thepresent invention by referring to the figures.

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings. Referring to FIGS. 4 and 5, asub-scanning interval adjusting apparatus 100 for a multi-beam laserscanning unit according to an embodiment of the present inventionincludes a transparent member 110, a movable member 120 and an elasticmember 130.

The transparent member 110 is made of a material, such as glass orplastics, through which a laser beam passes. The transparent member 110is generally formed by injection molding. The transparent member 110 hasa predetermined optical refractivity, and it is possible that therefractivity is more than 1. A cross-section of the transparent member110 may vary in thickness in accordance with a height direction, i.e., adirection (a sub-scanning direction as indicated with an arrow C)perpendicular to a laser scanning direction as indicated with an arrowB. It is possible that a lower portion is thicker while an upper portionis thinner. FIG. 5 shows one example of a structure of the transparentmember 110 in section, which has a shape of a trapezoid. Alternatively,the transparent member 110 may be a triangle in cross-section. Thetransparent member 110 has a lower side 111 formed at a predeterminedangle with respect to a lengthwise direction (the arrow B), i.e., thescanning direction.

The movable member 120 has an upper side 121 formed at an anglecorresponding to the angle of the lower side 111 of the transparentmember 110. The upper side 121 of the movable member 120 is slidablewith respect to the lower side 111 of the transparent member 110. Themovable member 120 is also provided with a driving unit (not shown)moving the movable member 120 leftward and rightward, i.e., in thescanning direction (the arrow B). Accordingly, as the movable member 120is moved leftward and rightward, the transparent member 110 is movedupward and downward (the arrow C).

The elastic member 130 is placed on the transparent member 110, pressingthe transparent member 110 with a predetermined pressure with respect tothe movable member 120. Being secured in a certain position of a frame135 of a scanning unit, the elastic member 130 also serves to secure thetransparent member 110 at a certain position in an optical path of thelaser beam. The elastic member 130 is made of any material that has aflexibility, and a compression spring may be used as the elastic member130.

An operation of the sub-scanning interval adjusting apparatus for themulti-beam scanning unit having the above structure, will be describedbelow.

When two laser lines are concentrated (formed) on a photoreceptor drum17 (FIG. 1) at an interval deviated from the predetermined interval, bydriving the driving unit, the movable member 120 is moved leftward orrightward, i.e., in the scanning direction (the arrow C). If the movablemember 120 is moved rightward, the transparent member 110 is movedupward by sliding along the inclined lower side 111. If the movablemember 120 is moved leftward, the transparent member 110 is moveddownward by sliding along the inclined lower side 111, and the downwardmovement of the transparent member 110 is carried out easily by arecovering force of the elastic member 130. As the transparent member110 is moved upward and downward with respect to the optical path of thetwo laser beams, the optical path changes by the transparent member 110,and as a result, the interval between the two laser lines can beadjusted.

A principle of adjusting the interval between the two laser linesthrough the upward and downward movement of the transparent member 110will be described below in greater detail.

FIG. 6 shows a process in which the optical path of the laser beamincident on the transparent member 110 changes as the laser beam passesthrough the transparent member 110. The laser beam passing through thetransparent member 110 satisfies the following equation of the Snell'slaw.sin θ1=n·sin θ2  Equation 1where n is a refractivity of the transparent member 110, θ1 is an angleof the laser beam being incident on the transparent member 110 withrespect to a surface perpendicular to an incident surface 110 a of thetransparent member 110, and θ2 is an angle of the laser beam having beenincident on the transparent member 110 with respect to the surfaceperpendicular to the incident surface 110 a of the transparent member110.

In FIG. 6, an upper laser beam of two laser beams emitted from two laserdiodes is indicated as a laser beam {circle around (1)}, while a lowerlaser beam is indicated as a laser beam {circle around (2)}. An intervalbetween the two incident laser beams {circle around (1)}, {circle around(2)} is L. When the optical path leads the two laser beams {circlearound (1)}, {circle around (2)} through the transparent member 110 chaving a uniform thickness, the interval between the laser beams {circlearound (1)}, {circle around (2)} emitted from the transparent member 110c is as much as L. However, when the optical path leads the two laserbeams {circle around (1)}, {circle around (2)} through the transparentmember 110 b having a variable thickness, the interval between the twolaser beams {circle around (1)}, {circle around (2)} emitted from thetransparent member 110 b is changed from L to L′. This is because ofdifferent optical distances of optical paths of the two laser beams{circle around (1)}, {circle around (2)} with respect to the a plane 110c parallel to the incident surface 110 a of transparent member 110, andan exit surface 110 b. An optical distance is obtained by multiplyingthe refractivity n of a medium by a thickness d of the medium.

Even with the transparent member 110 that is formed in the uniformthickness, a difference between the optical distances of the two laserbeams {circle around (1)}, {circle around (2)} occurs if the upper andlower laser beams fall onto the transparent member 110 in non-parallelrelation with each other.

FIGS. 7A and 7B show a changing interval between the two laser beamsbeing incident on the transparent member 110 that varies in thickness incross-section.

FIG. 7A shows a situation where the parallel laser beams are incident.The upper laser beam is indicated by {circle around (1)}, and the lowerlaser beam is indicated by {circle around (2)}. The upper and lowerlaser beams {circle around (1)}, {circle around (2)} are at an intervalL. With the transparent member 110 being at a position {circle around(s)}, the laser beams {circle around (1)}, {circle around (2)} passthrough the transparent member 110 and exit respectively from points{circle around (a)}, {circle around (b)}. The interval between the exitlaser beams is L′. Then when the transparent member 110 is moved by themovable member downward to another position {circle around (s)}′, thelaser beams {circle around (1)}, {circle around (2)} pass through thetransparent member 110 and exit respectively from points {circle around(a)}′, {circle around (b)}′. The interval between the exit laser beamsis L′. The optical path of the two laser beams varies by p2p3 and p2′p3′depending on and the position {circle around (s)}′. As noted from FIG.7A, a triangle p1p2p3 and another triangle p1′p2′p3′ are congruent, andthus, the optical paths p2p3 and p2′p3′ are identical in length.Accordingly, the interval L′ between the two laser beams can be keptconstant even when the thickness of the transparent member 110 varies.

FIG. 7B shows two non-parallel laser beams being incident on thetransparent member 110. The upper laser beam is indicated by {circlearound (1)} while the lower laser beam is indicated by {circle around(2)}. With the transparent member 110 being at the position {circlearound (s)}, the upper and lower laser beams {circle around (1)},{circle around (2)} pass through the transparent member 110 and exitrespectively from the points {circle around (c)}, {circle around (d)}.The interval between the exit laser beams is M. Then when thetransparent member 110 is moved by the movable member downward to theposition {circle around (s)}′, the laser beams {circle around (1)},{circle around (2)} pass through the transparent member 110 and exitrespectively from the points {circle around (c)}′, {circle around (d)}′.The interval between the exit laser beams is M′. The optical path of thetwo laser beams {circle around (1)}, {circle around (2)} varies by q2q3and q2′q3′ depending on the positioning of the transparent member 110 inposition {circle around (s)} and position {circle around (s)}′. As notedfrom FIG. 7B, a triangle q1q2q3 and another triangle q1′q2′q3′ aresimilar in certain proportion. Accordingly, the optical paths q2q3 andq2′q3′ are in the same proportion. That is, as the transparent member110 varies in thickness, the interval between the laser beams passingthrough the transparent member 110 also varies. Accordingly, by varyingthe transparent member 110 in thickness as described above, the intervalbetween the two laser lines on the photoreceptor drum is adjusted to apredetermined value.

FIGS. 8 and 9 shows an apparatus 200 adjusting an interval in asub-scanning direction for a multi-beam scanning unit according toanother embodiment of the present invention.

The sub-scanning interval adjusting apparatus 200 for the multi-beamscanning unit includes a transparent member 210, a movable member 220and an elastic member 230.

The transparent member 210 is made of a material, such as glass orplastics, through which a laser beam passes. The transparent member 210is generally formed by injection molding. The transparent member 210 hasa predetermined optical refractivity, and it is possible that therefractivity is more than 1. A cross-section of the transparent member210 may vary in thickness in accordance with a direction (sub-scanningdirection; arrow C) perpendicular to a height, i.e., to a laser scanningdirection (arrow B). It is possible that the lower portion is thickerwhile an upper portion is thinner. FIG. 9 shows one example of astructure of the transparent member 210 in cross-section, which has ashape of a trapezoid. Alternatively, the transparent member 210 may be atriangle in cross-section.

The movable member 220 is formed to push a lower side 211 of thetransparent member 210 upward in full-contact and provided with adriving unit (not shown) moving the movable member 220 upward anddownward, i.e., in the sub-scanning direction (arrow C). Accordingly, asthe movable member 220 is moved upward and downward, the transparentmember 210 is moved upward and downward (the arrow C). The driving unitmay be any proper device that can reciprocate the movable member 220 ina linear direction with accuracy. For example, precision screw devicescan be used as the driving unit.

The elastic member 230 is placed on the transparent member 210, pressingthe transparent member 210 with a predetermined pressure with respect tothe movable member 220. Being secured in a certain position of a frame235 of a scanning unit, the elastic member 230 also serves to secure thetransparent member 210 at a certain position in an optical path of thelaser beam. The elastic member 230 is made of any material that has aflexibility, and a compression spring can be used for the elastic member130.

An operation of the sub-scanning interval adjusting apparatus 200 forthe multi-beam scanning unit having the above structure, will bedescribed below.

When two laser lines are concentrated on a photoreceptor drum 17(FIG. 1) at an interval deviated from a predetermined laser line, bydriving the driving unit, the movable member 220 is moved upward anddownward, i.e., in the sub-scanning direction. If the movable member 120is moved downward, the transparent member 210 is also moved, causing thetransparent member 210 to be moved downward smoothly by a recoveringforce of the elastic member 230. As the transparent member 210 is movedupward and downward with respect to the optical path of the two laserbeams, the optical path changes by the transparent member 210, and as aresult, the interval between the two laser lines can be adjusted.

In accordance with the present invention as described above, theinterval between the two laser beams can be adjusted with ease, andwithout varying starting positions of the image formation of the laserbeams in the scanning direction.

Although a few preferred embodiments of the present invention have beendescribed, it will be understood by those skilled in the art that thepresent invention should not be limited to the described preferredembodiments, but various changes and modifications can be made withinthe spirit and scope of the present invention as defined by the appendedclaims and their equivalents.

1. A sub-scanning interval adjusting apparatus adjusting an interval between at least two laser beams for a multi-beam laser scanning unit, comprising: a transparent member arranged on an optical path of the at least two laser beams and which adjusts the interval between the at least two laser beams when moved, the transparent member having an inclined lower side inclined with respect to a scanning direction of the at least two laser beams; and a slidable transparent member mover which moves the transparent member when slid and has an inclined side corresponding to the inclined lower side of the transparent member.
 2. The sub-scanning interval adjusting apparatus of claim 1, wherein the transparent member comprises: a triangular shape in cross-section in the direction where the at least two laser beams pass through.
 3. The sub-scanning interval adjusting apparatus of claim 1, wherein the transparent member comprises: a trapezoidal shape in cross-section in the direction where the at least two laser beams pass through.
 4. A sub-scanning interval adjusting apparatus adjusting an interval between at least two laser beams for a multi-beam laser scanning unit, comprising: a transparent member having a thickness varying depending on a height thereof perpendicular to a direction where the at least two laser beams pass through and having an inclined lower side inclined with respect to a scanning direction of the at least two laser beams; and a slidable movable member which moves the transparent member in a height direction of the transparent member when slid, the slidable movable member having an inclined side corresponding to the inclined lower side of the transparent member, wherein the interval between the at least two laser beams is adjusted by varying the height of the transparent member with respect to the at least two laser beams.
 5. The sub-scanning interval adjusting apparatus of claim 4, wherein the transparent member comprises: a triangular shape in cross-section in the direction where the at least two laser beams pass through.
 6. The sub-scanning interval adjusting apparatus of claim 4, wherein the transparent member comprises: a trapezoidal shape in cross-section in the direction where the at least two laser beams pass through.
 7. The sub-scanning interval adjusting apparatus of claim 4, wherein the transparent member has an optical refractivity equal to or great than
 1. 8. The sub-scanning interval adjusting apparatus of claim 4, further comprising: an elastic member pressing the transparent member against the movable member.
 9. The sub-scanning interval adjusting apparatus of claim 4, wherein the transparent member is made either of glass or plastic by injection molding.
 10. A sub-scanning interval adjusting apparatus adjusting an interval between at least two laser beams for a multi-beam laser scanning unit, the apparatus comprising: a transparent member having a thickness varying depending on a height thereof perpendicular to a direction where the at least two laser beams pass through, and having an inclined lower side inclined with respect to a scanning direction of the at least two laser beams; and a movable member having an inclined side corresponding to the inclined lower side of the transparent member, wherein the interval between the at least two laser beams is adjusted by moving the movable member in the scanning direction and thus varying a height of the transparent member with respect to the at least two laser beams.
 11. The sub-scanning interval adjusting apparatus of claim 10, wherein the transparent member comprises: a triangular shape in cross-section in the direction where the at least two laser beams pass through.
 12. The sub-scanning interval adjusting apparatus of claim 10, wherein the transparent member comprises: a trapezoidal shape in cross-section in the direction where the at least two laser beams pass through.
 13. The sub-scanning interval adjusting apparatus of claim 10, wherein the transparent member has an optical refractivity equal to or greater than
 1. 14. The sub-scanning interval adjusting apparatus of claim 10, further comprising: an elastic member pressing the transparent member with respect to the movable member.
 15. The sub-scanning interval adjusting apparatus of claim 10, wherein the transparent member is made either of glass or plastic by injection molding.
 16. A sub-scanning interval adjusting apparatus adjusting an interval between first and second laser beams respectively emitted by first and second diodes along respective first and second optical paths spaced apart from each other in a multi-beam laser scanning unit, comprising: a transparent member disposed on the first and second optical paths of the first and second laser beams, and having a first thickness corresponding to the first optical path of the first laser beam, a second thickness corresponding to the second optical path of the second laser beam in an optical path direction of the first and second optical paths, and an inclined lower side inclined with respect to a scanning direction of the first and second laser beams; and a slidable moving member which controls the transparent member to adjust the first thickness and the second thickness of the transparent member when slid so as to adjust the interval between the first and second laser beams, the slidable moving member having an inclined side corresponding to the inclined lower side of the transparent member.
 17. The sub-scanning interval adjusting apparatus of claim 16, wherein the first and second laser diodes simultaneously emit the first and second laser beams toward the transparent member.
 18. The sub-scanning interval adjusting apparatus of claim 16, wherein the first and second laser diodes simultaneously emit the first and second laser beams parallel to each other by an initial interval which is changed to the interval by the transparent member.
 19. The sub-scanning interval adjusting apparatus of claim 16, wherein the moving member moves the transparent member in a direction perpendicular to a line passing through the first and second optical paths to change the first thickness and the second thickness, and the first and second laser diodes are disposed in the direction perpendicular to the line passing through the first and second optical paths.
 20. The sub-scanning interval adjusting apparatus of claim 16, wherein the first and second laser beams form an initial interval in a direction perpendicular to the optical path direction of the first and second optical paths when being incident to the transparent member and form the interval greater than the initial interval when exiting the transparent member.
 21. The sub-scanning interval adjusting apparatus of claim 16, wherein the first thickness and the second thickness of the transparent member are not the same.
 22. The sub-scanning interval adjusting apparatus of claim 16, wherein the first thickness and the second thickness simultaneously increase or decrease when the moving member moves the transparent member in a direction having an angle with the optical path direction of the first and second optical paths.
 23. The sub-scanning interval adjusting apparatus of claim 16, further comprising: a frame on which the transparent member is movably disposed.
 24. The sub-scanning interval adjusting apparatus of claim 23, further comprising: an elastic member coupled between the transparent member and the frame to bias the transparent member with respect to the frame.
 25. The sub-scanning interval adjusting apparatus of claim 24, wherein the frame, the transparent member, and the moving member are disposed in a direction perpendicular to the optical path direction.
 26. The sub-scanning interval adjusting apparatus of claim 24, further comprising: a driving unit moving the moving member to move the transparent member in a direction having an angle with the optical path direction.
 27. The sub-scanning interval adjusting apparatus of claim 16, wherein the transparent member narrows in a direction having an angle with the optical path direction.
 28. The sub-scanning interval adjusting apparatus of claim 16, wherein the transparent member moves in a direction having an angle with the optical path direction to change the first thickness and the second thickness in response to a movement of the moving member.
 29. The sub-scanning interval adjusting apparatus of claim 16, wherein the transparent member comprises: incident and exit surfaces disposed on the first and second optical paths to be spaced-apart from each other by a variable thickness including the first thickness and the second thickness.
 30. A sub-scanning interval adjusting apparatus adjusting an interval between first and second laser beams respectively emitted by first and second diodes along respective first and second optical paths spaced apart from each other in a multi-beam laser scanning unit, comprising: a transparent member disposed on the first and second optical paths of the first and second laser beams, and having a first thickness corresponding to the first optical path of the first laser beam and a second thickness corresponding to the second optical path of the second laser beam in an optical path direction of the first and second optical paths; a slidable moving member which controls the transparent member to adjust the first thickness and the second thickness of the transparent member when slid so as to adjust the interval between the first and second laser beams; a frame on which the transparent member is movably disposed; and an elastic member coupled between the transparent member and the frame to bias the transparent member with respect to the frame, wherein the moving member is disposed opposite to the elastic member with respect to the transparent member to move the transparent member with respect to the frame and the elastic member.
 31. A sub-scanning interval adjusting apparatus adjusting an interval between first and second laser beams respectively emitted by first and second diodes along respective first and second optical paths spaced apart from each other in a multi-beam laser scanning unit, comprising: a transparent member disposed on the first and second optical paths of the first and second laser beams, and having a first thickness corresponding to the first optical path of the first laser beam and a second thickness corresponding to the second optical path of the second laser beam in an optical path direction of the first and second optical paths; and a slidable moving member which controls the transparent member to adjust the first thickness and the second thickness of the transparent member when slid so as to adjust the interval between the first and second laser beams, wherein the transparent member includes a first inclined surface inclined with respect to the first and second optical paths and the moving member includes a second inclined surface corresponding to the first inclined surface of the transparent member.
 32. The sub-scanning interval adjusting apparatus of claim 31, wherein the first inclined surface and the second inclined surface are disposed in a direction having an angle with the optical path direction of the first and second optical paths.
 33. The sub-scanning interval adjusting apparatus of claim 31, wherein the transparent member moves in a direction perpendicular to the optical path direction when the moving member moves in the optical path direction.
 34. The sub-scanning interval adjusting apparatus of claim 31, wherein the first inclined surface of the transparent member slides along the second surface of the moving member in a direction having an angle with the optical path direction.
 35. A sub-scanning interval adjusting apparatus adjusting an interval between first and second laser beams respectively and simultaneously emitted by first and second diodes along respective first and second optical paths spaced apart from each other in a multi-beam laser scanning unit, the apparatus comprising: a transparent member disposed on the first and second optical paths of the first and second laser beams, narrowing in a direction having an angle with an optical path direction of the first and second optical paths, and having an inclined lower side inclined with respect to a scanning direction of the at least two laser beams; and a slidable moving member which moves the transparent member when slid so as to adjust the interval between the first and second laser beams, the slidable moving member having an inclined side corresponding to the inclined lower side of the transparent member.
 36. A multi-beam laser scanning unit having an interval between two laser beams, the multi-beam laser scanning unit comprising: a sub-scanning interval adjusting apparatus comprising a transparent member disposed on the first and second optical paths of first and second laser beams respectively emitted by first and second diodes along respective first and second optical paths spaced apart from each other to narrow in a direction having an angle with an optical path direction of the first and second optical paths and having an inclined lower side inclined with respect to a scanning direction of the at least two laser beams, and a slidable moving member which moves the transparent member when slid so as to adjust the interval between the first and second laser beams, the slidable moving member having an inclined side corresponding to the inclined lower side of the transparent member; and a photoreceptor drum on which the first and second laser beams form first and second scan lines spaced-apart from each other by a distance corresponding to the interval.
 37. The multi-beam laser scanning unit of claim 36, further comprising: a polygon mirror disposed between the sub-scanning interval adjusting apparatus and the photoreceptor drum to direct the first and second laser beams toward the photoreceptor drum, respectively.
 38. The multi-beam laser scanning unit of claim 37, further comprising: an f-θ lens disposed between the polygon mirror and the photoreceptor drum to focus the first and second laser beams on the photoreceptor drum.
 39. The multi-beam laser scanning unit of claim 36, wherein the first and second laser diodes simultaneously emit the first and second two laser beams, respectively, and the first and second scanning lines are simultaneously formed on the photoreceptor drum.
 40. The multi-beam laser scanning unit of claim 36, wherein the first and second scanning lines are formed in a scanning direction, and the first and second scanning lines are spaced-apart from each other in a sub-scanning direction perpendicular to the scanning direction.
 41. The multi-beam scanning unit of claim 36, wherein the transparent member comprises: a first thickness corresponding to the first optical path of the first laser beam in the optical path direction; and a second thickness corresponding to the second optical path of the second laser beam in the optical path direction.
 42. The multi-beam scanning unit of claim 36, wherein the first scanning line of the first laser beam has a first starting point of the photoreceptor corresponding to a reference point, and the second scanning line of the second laser beam has a second starting point corresponding to the reference point.
 43. The multi-beam scanning unit of claim 36, wherein the first and second laser beams are disposed in parallel in the direction perpendicular to the optical path direction of the first and second optical paths.
 44. The multi-beam scanning unit of claim 36, wherein the first and second start points of the first and second scanning lines do not vary when the distance of the first and second scanning lines varies according to adjustment of the interval of the first and second laser beams. 